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An Introduction to Cyanidation

The cyanide process has allowed the development of large, low grade
precious metal deposits more than any other process.The early development of the process is attributed to a
Scotsman, John Stewart MacArthur, in collaboration with Dr Robert
and Dr William Forrest. The method was introduced into South Africa
in 1890. From there it
spread to Australia, Mexico and the United States. Now it is used in
practically all the major gold mining areas of the world.

The reasons for its widespread acceptance are economic as
well as metallurgical. It usually obtains a higher gold recovery
than other processes, especially for fine gold.
Cyanidation is easier to
operate than the other leaching processes, and is safer to use than
mercury amalgamation. It produces the final product in the form of
practically pure metal. Thus the production from a large cyanide
mill will be represented by a comparatively small gold bar, which is
easy to transport. Accordingly gold mines can be located in
relatively inaccessible districts served only by airplane or truck.

However, the gold metallurgist must be familiar with the
other processes of gold treatment, such as gravity concentration and
flotation (and, in some locations still, amalgamation), as they are
frequently used as an auxiliary to the cyanide process.

General
Theory

Gold does not oxide (tarnish) at ordinary temperatures nor is it
soluble in sulphuric, nitric or hydrochloric acids. It does dissolve
in aqua regia (a mixture of nitric and hydrochloric acid) also in
some chlorine and bromine compounds. The chlorine and bromine reaction was the base for the
bromo-cyanide method used on some refractory ores in the early days
of gold mining in Australia. Gold
is also soluble in mercury, uniting with it to form amalgam.
However, the main chemical property of commercial interest is that
gold is soluble in dilute cyanide solutions.

The basis of the cyanide process is that weak solutions of
sodium or potassium cyanide have a preferential dissolving action on
small particles of metallic gold and silver over other materials
common in gold ores. However, there are a few minerals known as
cyanicides that have deleterious effects which are discussed later.

Cyanide is the general descriptive term applied usually to
sodium cyanide (NaCN). However, the early work in cyanidation was
based on the use of potassium cyanide and strengths of solution as
well as basic formulae are still in terms of that chemical.It is to be noted that the cyanogen radical (CN) actually has
the dissolving power, the alkaline base of potassium, calcium or
sodium merely giving a chemical stability to the compound.

The main difference between the alkaline cyanides, aside from
their cost, is their relative dissolving power. This depends
entirely on the percentage of cyanogen radical present.

Strength of solution is usually about one pound of cyanide to a ton
solution (water) (or 0.02% -0.05% NaCN).
This is usually sufficiently strong for most straight cyanide
circuits and experimental work has shown that maximum dissolving
power is obtained at this strength. Furthermore, a weak solution is
less affected by cyancides, and danger of poisoning from fumes
formed by evaporation in hot weather is decreased.

Gold sulphide concentrates, obtained by table concentration
or flotation, are frequently treated by higher strength solutions.
These concentrates usually require very thorough study.

Strength of solutions is usually expressed in pounds of
equivalent potassium cyanide per ton of solution. 1 lb. cyanide to 1
ton of water = 0.05% solution; 2
lb cyanide = 0.10%, etc.

Temperature of solution is also important in maintaining efficient
dissolving action. Especially in cold climates, the solutions are
frequently heated to about 70°F. Above this temperature the loss of
cyanide by decomposition becomes a serious factor Theoretically,
gold dissolves fastest in a solution at a temperature of 138°F.

Slurry
(Pulp) Density

To maintain maximum capacity and minimum loss of valuable material
in solution, it is usually advisable to maintain the highest
densities in the mill circuits. It should be kept in mind that for
every ton of water added to the mill circuit, a ton of water ends up
in the tailings, which needs to be handled.This tailings solution not only contains reagents, such as
lime and cyanide, but also dissolved gold, even if only in minute
quantities.

The higher the density of the feed to the leach circuit the greater
the capacity of the circuit, or conversely, smaller or fewer tanks
and agitators are required. Assuming an ore where the solids have a
specific gravity of 2.6, one ton of solids as 30% solids (70%
solution) will occupy 86.7 cubic feet, while at 50% solids will only
occupy about half that space, namely, 44.3 cubic feet. Also there is
apt to be more settling of sand fractions which may cause mechanical
difficulties when treating a dilute pulp. Accordingly agitator
densities are usually kept between 30% to 60% solids.
Grinding capacity in a ball
mill is also limited if the density drops below 70% solids.

Aeration

Another of the prime requisites of successful cyanidation is free
oxygen. For efficient
dissolving, it is necessary that the oxygen (air) come in actual
physical contact with the gold particles. As these particles are
usually very sparsely distributed through the pulp, it means that
the air bubbles should be thoroughly dispersed and a huge excess be
used beyond theoretical requirements of air.

Oxidizing agents have also been used. These oxidizers may be
sodium peroxide, potassium permanganate or manganese dioxide. They
act in two ways; by a nascent or active condition and so
accelerating the dissolving of gold, and by oxidizing impurities
that may be present in the ore or solution.

It has been found in some mills that due to increases in tonnages or
changes in the ore, that extra aeration is necessary. Various
methods have been used for this, such as placing a ring of air jets
around the circumference of each of the agitators. High pressure
compressed air was used, giving violent aeration and agitation.
Another method is based on
the dispersal of the flow of pulp as it enters the various tanks in
one large thick stream. Another
method noted in the field is the insertion through the sides of the
tank, well below the top of the pulp.

Decomposition
of Reagents

The amount of reagents actually required for dissolving the gold is
extremely small. However, frequently the amount of reagents used is
much higher and certain causes should be recognized and, if
possible, remedied. These may be briefly listed as follows:

·
Water Quality

·
Cyanicides

·
Mechanical losses

Water Quality

The source of water is very important, not only from the viewpoint
of the quantity, but also the quality. In some districts the only
water available is from small lakes or ponds and, as such, is
frequently contaminated with organic material and soluble salts.
This water may be highly reducing in its action. Extra lime
treatment may be necessary before this water joins the mill return
stream. Flocculants and percipatates may be added to aid in the
precipitation of the soluble salts. Chemical oxidizers such as
potassium permanganate are also used. Some of these problems are
also discussed further under the heading of clarification.

Cyanicides

Certain materials known as cyanicides may be present in the ore. A
cyanicide may be defined as a natural occurring material that
destroys cyanide.
Pyrrhotite is one of the best known. It combines with the cyanide
giving ferro-cyanide and sulphocyanide.It is stated that stibnite requires extremely low alkalinity
to prevent its solubility in solution.The reverse is true in the case of sphalerite, where high
lime tends to reduce zinc solubility.Ores often contain copper, antimony, arsenic, cobalt or
nickel sulphides which go into solution through the action of the
cyanide.

Although the dissolving rate of these materials may be controlled to
some extent, the solutions in time will lose their potency due to
being re-cycled.Then
it is necessary to bleed off part of the fouled solution and restore
the balance by the addition of fresh stock. After the solutions have
been de-aerated and precipitated, it is also necessary to positively
aerate them before being used. This
aeration not only restores the free oxygen to the solution, but also
partially regenerates some of the combined cyanide.

Mechanical losses

They occur in two ways:

(a) Accidental losses.

(b) Inherent losses.

The
first is due to spills and leaks on account of poor launder design
and tank spillages. Losses also occur when it is necessary to dump
agitators, classifiers or thickener tanks, due to power failures or
mechanical difficulties.

Inherent
losses may also be considered from two view-points, namely, those
occurring only in a new circuit, and those occurring continuously.
The first is due to solutions soaking into the fresh wood tanks and
may occur over a period of two or three months. The second is due to
losses from filter discharges, etc. Naturally it is desired to keep
these losses at a minimum, as they are a flat charge against the
cost of operation. For example, a filter cake may have 10 to 12'10
moisture in it. A heavy water wash on the filter will reduce the
amount of chemicals in this moisture, while re-pulping and second
filtration may be advisable in some cases. A cost analysis in every
case is desirable.

pH
Control

To improve metal recovery and to reduce the amount of cyanide
needed, the slurry pH is adjusted, normally by adding lime to
maintain a "protective alkalinity." It is usual to keep this
alkalinity at from ½ to 1 ½ Ibs. per ton of solution. Lime has a
further beneficial effect of hastening settlement of finely ground
rock, or slimes, in thickeners, and it further precipitates certain
undesirable substances.

In order that the lime will begin its protective action as
soon as possible, it is usually added with the fresh ore in the ball
mill (if the ore is ground). It may be added dry or as a milk of
lime. Frequent and systematic sampling of mill solutions at various
predetermined points in the dissolving circuit is advisable. Then
the operator can control the lime and cyanide strength and be
certain at all times that minimum required strength is being
maintained. Cyanide is usually added in the freshly aerated solution
pumped to the grinding circuit, although sometimes blocks of cyanide
may be suspended in baskets in a dissolving circuit to remedy some
local trouble.